Method of manufacturing a capacitive transducer

ABSTRACT

A method of manufacturing a capacitive transducer by applying a first etching mask on a layer. Applying a second etching mask to define the movable set of fingers, the fixed set of fingers, a body, and springs, and the body is connected to the movable set of fingers and the springs while the movable set of fingers are interdigitated with the fixed set of fingers. Etching the layer and the first etching mask using the second etching mask and removing the second etching mask. Etching the layer such that one of the movable set of fingers and the fixed set of fingers is shorter than the other of the movable set of fingers and the fixed set of fingers. Releasing the body, the springs, and the movable set of fingers using etching, such that, upon applying a force to the body, the body moves parallel to the substrate.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 11/946,204 filed Nov. 28, 2007 now U.S. Pat. No. 8,165,323.

FIELD

The present patent document relates to a monolithic capacitivetransducer, such as a miniature capacitive transducer.

BACKGROUND

Miniature micromachined microphones have gained great popularity in avariety of applications. Because of its sub-mm size, low cost for massproduction, lower power consumption, higher sensitivity and reliability,it is widely recognized as the next generation product to replace theconventional electrets condenser microphone (ECM) in applications suchas hearing aids, cell phones, PDAs, laptops, MP3, digital cameras etc.Among all the micromachined microphones, the capacitive condenser typeof microphone has many advantages over other technical approaches suchas piezoelectric or magnetic type micromachined microphone for itssmaller size and higher sensitivity etc.

The micromachined condenser microphone typically consists of an acousticpressure sensing element, generally a variable capacitor, and apreamplifier IC circuit. One prior art example of a condensermicrophones with a parallel plate capacitor is disclosed in U.S. patentpublication no. 2006/0093170 (Zhe et al.) entitled “Backplatelesssilicon microphone”. The prior art suffers from some or all ofshortcomings mentioned below due to the structure and sensing motion ofthe parallel plate variable capacitor.

First of all, residual film stress on the diaphragm reduces thesensitivity of the microphone. Since the compliant diaphragm is usuallymade of a thin film of dielectric and electrically conductive materials,it is very difficult to control or reduce its residual stress becausethe residual stress is present after the film formation. Stress on thediaphragm has a direct impact on the sensitivity of the microphone.Compressive residual stress results in a defective, buckled diaphragm.Tensile stress severely decreases the sensitivity of the microphone, ortotally ruptures the diaphragm at the worst cases.

Secondly, stiction between a flexible diaphragm and a rigid backplatecan result in either a faulty device during microfabrication ormalfunction during operation. When the gap between the compliantdiaphragm and the backplate is on the order of several microns, thediaphragm will adhere to the fixed backplate with a larger probabilitybecause the surface to volume ratio increases and surface forces, whichare responsible for stiction, are correspondingly higher. Stiction couldprevent the successful releasing of the suspended compliant diaphragmduring the wet process of the sacrificial layer etching, leading topermanent adhesion to the fixed backplate. During the operation, if themicrophone is exposed to a humid environment, water vapor can condenseand form a water film on the diaphragm and backplate surfaces. When thegap between the two surfaces decreases during operation and the waterfilm of one surface touches the counter surface, the two surfaces willstick together.

Thirdly, “squeeze film” air damping affects the high frequency response,and contributes noise to the microphone output by generating pressurefluctuations in the microphone structure. For the sub-mm-sizedcapacitive condenser microphone, the air gap must be scaled down toseveral microns to keep the capacitance value in a range which can drivethe input of the buffer amplifier effectively. However, as the air gapis reduced, the “squeeze film” damping effects due to the viscous flowof air trapped between the diaphragm and backplate increases rapidly.“Squeeze film” air damping can also impact the sensitivity of themicrophone.

Fourthly, the “pull-in” effect of the diaphragm reduces the DC biasvoltage, which therefore lowers the sensitivity of the microphone. Ahigher DC bias voltage between diaphragm and backplate yields highersensitivity. A higher DC bias voltage will create a larger attractiveelectrostatic force between the diaphragm and backplate. However, insome prior art examples, the gap between the diaphragm and backplate isreduced to several microns, and the mechanical compliance of thediaphragm is kept fairly low in order to have some deflection undercertain sound pressure level. Larger attractive electrostatic force canovercome the mechanical restoring force of the diaphragm, and can pullthe compliant diaphragm over the small gap to touch the backplate. Thisphenomenon is called the “pull-in” effect.

Fifthly, a sub-mm-sized diaphragm that is fully constrained by thesurrounding frame reduces the sensitivity of the microphone. Thecompliance of the diaphragm tends to decease very rapidly with thedecreasing size for a given diaphragm material and thickness. Themechanical compliance/stiffness of the diaphragm for the sound pressurescales as the fourth power of the diaphragm size.

Sixthly, the small air gap and compliant diaphragm of parallel platetype capacitive condenser microphones can't provide a large dynamicrange as higher sound pressure levels could drive the flexible diaphragmto contact the backplate across the small air gap.

Seventhly, the parasitic capacitance between the flexible diaphragm andrigid fixed backplate degrades the microphone performance. Thecapacitance between the diaphragm and backplate has two parts. The firstpart varies with acoustical signal and is desirable for microphone. Thesecond part is a parasitic capacitance which does not vary withacoustical signal. The parasitic capacitance degrades the performanceand should be minimized. However, the parasitic capacitance is relatedto the construction of the parallel plate type of silicon microphone inthe prior arts.

Last but not least, the parallel plate type capacitive condensermicrophone is fairly complicated and costly for manufacturing. So far,the prior art has been unable to provide an economic manufacturingmethod for the mass production of microphones. Some manufacturingmethods of sensing elements disclosed in the prior art are notcompatible with standard IC CMOS process, resulting in larger hybridpackage and higher manufacturing cost.

SUMMARY

A capacitive transducer includes a substrate having a first surface anda second surface. The first surface of the substrate defines a firstplane. The substrate has a cavity with an interior peripheral edge. Thecavity extends between the first surface and the second surface. A bodyis provided that has an exterior peripheral edge. The body is parallelto the first plane and at least partially blocking the cavity. The bodyis connected to the substrate by resilient hinges such that, upon theapplication of a force, the body moves perpendicular to the first plane.A first set of comb fingers is mounted to the substrate. The first setof comb fingers is connected to a first electrical connection. A secondset of comb fingers is mounted to the body and extends past the exteriorperipheral edge of the body. The second set of comb fingers is connectedto a second electrical connection that is isolated from the firstconnection. The first set of comb fingers and the second set of combfinger are interdigitated such that as the body moves, the first set ofcomb fingers and the second set of comb finger maintain a relativespacing. The first set of comb fingers and the second set of combfingers define a capacitance. The capacitance is related to the relativeposition of the first set of comb drive fingers and the second set ofcomb drive fingers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will become more apparent from the followingdescription in which reference is made to the appended drawings, thedrawings are for the purpose of illustration only and are not intendedto be in any way limiting, wherein:

FIG. 1 is a cross sectional perspective view of the microphone accordingto a first embodiment;

FIG. 2 is a cross sectional view showing the structure of the SOI waferfor the transducer according to a first embodiment;

FIG. 3 is a cross sectional view of the SOI wafer after deposition oflayer of oxide on its top and bottom sides according to a firstembodiment;

FIG. 4 is a cross sectional view of the SOI wafer after anisotropicsilicon etching of backside cavity and oxide etching from its backside;

FIG. 5 is a perspective view of the SOI wafer after patterning andetching the oxide on its front side according to a first embodiment;

FIG. 6 a is a perspective view of the SOI wafer in FIG. 5 afterpatterning of a layer of photoresist;

FIG. 6 b is an enlarged perspective view of a portion C of the combfingers and hinge depicted in FIG. 6 a;

FIG. 7 a is a perspective view of the transducer in FIG. 6 afterReactive Ion Etching (RIE) of a layer of oxide;

FIG. 7 b is an enlarged perspective view of a portion D of the combfingers and hinge depicted in FIG. 7 a;

FIG. 8 is a perspective view of the transducer in FIG. 7 after the firstDeep Reactive Ion Etching (DRIE) of silicon;

FIG. 9 a is a perspective view of the transducer in FIG. 8 after removalof photoresist;

FIG. 9 b is an enlarged perspective view of a portion E of the combfinger and hinge depicted in FIG. 9 a;

FIG. 10 a is a perspective view of the transducer in FIG. 9 after thesecond Deep Reactive Ion Etching (DRIE) of silicon;

FIG. 10 b is an enlarged perspective view of a portion F of the combfinger and hinge depicted in FIG. 10 a;

FIG. 11 is a perspective view of the transducer in FIG. 10 after removalof oxide on the front side and partial etching of buried oxide layer torelease the diaphragm and movable fingers;

FIG. 12 is a cross sectional perspective view of the microphoneaccording to a second embodiment;

FIG. 13 is the top view of the microphone depicted in FIG. 1 and FIG.12;

FIG. 14 is a cross sectional perspective view of the microspeaker with alarger back cavity and taller comb finger according to a thirdembodiment;

FIG. 15 is a cross sectional perspective view of the microspeaker with alarger back cavity and taller comb finger according to a fourthembodiment;

FIG. 16 is a cross sectional perspective view of the microspeaker with alarger back cavity and taller comb finger according to a fifthembodiment;

FIG. 17 is a cross sectional perspective view of the N type substratefor the transducer according to a sixth embodiment;

FIG. 18 is a cross sectional perspective view of the N type substrateafter P++ implantation/diffusion, or epitaxial growth of layer a siliconwith P doping according to a sixth embodiment;

FIG. 19 is a cross sectional perspective view of the substrate depictedin FIG. 18 after anisotropic silicon etching of backside cavityaccording to a sixth embodiment;

FIG. 20 is a cross sectional perspective view of the transducer in FIG.19 after process of silicon Deep Reactive Ion Etching (DRIE) with theself-align process disclosed herein.

DETAILED DESCRIPTION

The device described below is a miniature sub-mm-sized capacitivecondenser with higher sensitivity, larger dynamic measurement range thatovercomes the disadvantages of the parallel plate condenser siliconmicrophones disclosed in the prior art. The sensing element structure ofthe microphone reduces or eliminates residual stress effects, stiction,“squeeze film” air damping, and “pull-in”. This kind of transducer maybe used in microphones and microspeakers used for hearing aids, cellphones, PDAs, laptops, MP3 players, digital cameras and otherapplications. It may also be used as an accelerometer, pressure sensors,actuator for a pump, optical switches, and optical interferometers. Thedesign and fabrication method described below can also be used forminiature low voltage electrostatic driven microspeakers,accelerometers, etc. In one embodiment, the manufacturing method for thesensing and actuation structure is compatible with standard IC COMSprocess to form a monolithic integrated miniature silicon capacitivetransducer.

The vertical combdrive structure that allows sensing or actuationeliminates residual thin film stress on the diaphragm, “pull-in” effectsand “squeeze film” air damping of the parallel plate type of capacitivesensing and actuation in the prior art examples. The working capacitanceof the device is achieved by the interdigital vertical comb fingers. Thevertical comb finger structure avoids the need of the backplate whichattributes microfabrication challenges and performance sacrifices. Theteachings herein provides design and microfabrication method for bothsub-mm-sized silicon capacitive microphone with higher sensitivity,larger dynamic range, a miniature electrostatic driven microspeaker withlow power consumption and low driving voltage, and also a miniaturecapacitive accelerometer etc.

The same structure design principle can be used either in the sensingmode which is applicable for a microphone or an accelerometer etc., orin the actuation mode which is applicable for a microspeaker etc.

Sensing Mode

FIGS. 1, 11 and 12 show a suitable example of a device structure designused for sensing, which is useful, for example, as a microphone oraccelerometer. In this embodiment, the device has been formed using aSOI (Silicon On Insulator) substrate. The capacitive transducer is madefrom bulk conductive silicon mounted on a substrate, which is alsoreferred to below as a carrier wafer 12. The diaphragm 32 is supportedby four hinges 29 a, 29 b, 29 c and 29 d that are connected to thecorners of the rectangular diaphragm 32. The diaphragm 32, which is thebody that moves when a force is applied, is connected to the fixedanchors 37 a, 37 b, 37 c and 37 d mounted on the substrate, which is aby serpentine shaped silicon hinges 29 a, 29 b, 29 c and 29 d,respectively. The anchors 37 a, 37 b, 37 c and 37 d are sitting on adielectrical material layer 11, such as an oxide. The sensing element ismade up of a vertical combdrive structure that includes a first set offixed comb fingers 35 and a second set of movable lower comb fingers 36.All the movable comb fingers 36 are formed on the outside edge of thediaphragm 32. It will be understood that comb fingers 35 and 36 need notbe positioned on all sides of the diaphragm 32 as shown. For example,the fingers may be positioned on two parallel edges of the diaphragm 32.The fixed comb finger structures 35 are built around the diaphragm 32and fixed on the dielectrical material 11 by anchors 38 a, 38 b, 38 cand 38 d. The diaphragm 32, hinges 29 a, 29 b, 29 c and 29 d, anchors 37a, 37 b, 37 c, 37 d, 38 a, 38 b, 38 c and 38 d, vertical comb fingers 35and 36 and electrical interconnection structures 39 a, 39 b and 39 c aremade of the same layer of electrical conductive material, such as aconductive single crystal silicon 10 which is on the top of a layerdielectrical material 11 that separates the silicon layer 10 from a basesubstrate 12 in a SOI structure. The electrical interconnectionstructures 39 a, 39 b and 39 c electrically connect all four fixed combfinger structures 35 which are around the diaphragm 32, while movablecomb finger structures 36 are electrically connected by diaphragm 32. Assuch, any anchor 37 a, 37 b, 37 c, and 37 d, can be used as anelectrical connection point, and any anchor 38 a, 38 b, 38 c and 38 dcan be used as the other electrical connection point for an integratedon-chip IC circuit, or for the wire bonding pads if a hybrid package isrequired for the transducer.

As can be seen in FIG. 1, the exterior peripheral edge of the diaphragm32 overlaps the interior peripheral edge of the cavity 40. When used asa microphone, this overlap of the diaphragm 32 and the carrier wafer 12is required to create a long air flow path 33 between the diagram 32with movable fingers 36 and carrier wafer 12 to reduce leakage aroundthe diaphragm 32. This establishes a reasonably high resistance, whichimproves the low frequency response of the transducer. Another method ofreducing leakage is to coat the diaphragm 32 on the cavity side 40 witha light weight material, such as a polymer (not shown) to reduce the gapbetween the diagram 32 with movable fingers 36 and carrier wafer 12.This may be done, for example, by sputtering or other depositiontechniques. During deposition, the material may also be deposited on thesides of the cavity. However, this is not undesirable, as it would alsoreduce the gap.

The capacitance increases with the number of comb fingers. For asub-mm-size diaphragm, it is possible to form a sufficient number ofmovable comb fingers 36 to achieve working capacitance of Pico Farads,which is equivalent to the capacitance offered by parallel platestructure in the prior arts. When the diaphragm 32 is subjected topressure waves such as acoustic pressure, oraccelerations/decelerations, the diaphragm 32 will move up and down in apiston style movement. The serpentine design of the springs 29 a, 29 b,29 c, and 29 d helps establish a substantially linear movementthroughout. The movement of the diaphragm 32 can be detected bymonitoring the capacitance changes between the movable fingers 36 andfixed comb fingers 35. In addition, the capacitance changes between themovable fingers 36 and the fixed carrier wafer 12 may also be measured,which for example increases the sensitivity of the sensor by taking adifferential measurement of the changes in capacitance. In addition,since the vertical comb finger structure and flexible hinges are used,the capacitance change is more sensitive to acoustic pressure 34 oraccelerations/decelerations due to the fringe effects of the small combfingers, which also results in higher sensitivity of the transducer. Theflexible hinges helps maintain the piston movement of the diaphragm 32instead of the parabolic deformation of the diaphragm in the prior art.

The etching cavities 20 a, 20 b, 20 c and 20 d on the diaphragm 32 areto reduce the mass the diaphragm 32 for better high frequency response.The transducer requires no backplate, since its diaphragm 32 issuspended on the cavity 40 of the carrier wafer 12. A barometric reliefis not needed for the microphone.

Actuation Mode

FIGS. 14, 15 and 16 show a device that is designed to be used in theactuation mode, such as for a microspeaker. Similar reference numbersare used to the embodiment described above. A silicon capacitivetransducer (microspeaker) comprises a diaphragm 32 supported by fourhinges 29 a, 29 b, 29 c and 29 d. The diaphragm 32 is made of the bulkconductive silicon connected to the fixed anchors by serpentine shapedsilicon hinges 29 a, 29 b, 29 c and 29 d. The four hinges are connectedto the anchor 37 a, 37 b, 37 c and 37 d which are sitting on thedielectrical material 11. The actuation element is the verticalcombdrive structure and includes movable comb fingers 36 and fixed combfingers 35. The movable comb fingers 36 are formed on the outside edgeof the diaphragm 32. The fixed comb finger structures 35 are builtaround the diaphragm 32 and fixed on the dielectrical material 11 byanchor 38 a, 38 b, 38 c and 38 d. The diaphragm 32, hinges 29 a, 29 b,29 c and 29 d, anchors 37 a, 37 b, 37 c, 37 d, 38 a, 38 b, 38 c and 38d, vertical comb fingers 35 and 36 and electrical interconnectionstructure 39 a, 39 b and 39 c are made of the same layer of electricalconductive silicon 10 which is on the top of dielectrical material 11.The electrical interconnection structure 39 a, 39 b and 39 celectrically connects all four fixed comb finger structures 35 aroundthe diaphragm 32. Anchor 37 and 38 can be used as the electricalconnection points for the integrated on-chip IC circuit, or for thebonding pads if a hybrid package is required for the transducer. Forsub-mm-size or mm-size diaphragm, sufficient number of movable combfingers 36 can be formed on its edge to achieve working capacitance ofPico Farads which is equivalent to the capacitance offered by parallelplate structure in the prior arts. When the actuation voltage is applybetween the anchor 37 and 38, a high density electrical field will becreated between the fixed comb fingers 35 and movable fingers 36. Theresulting electrostatic force will actuate the diaphragm 32 to createacoustic pressure wave. The flexible hinges 29 a, 29 b, 29 c and 29 dwill maintain the piston movement of the diaphragm 32 instead of theparabolic deformation which is common to many prior art devices. Theetching cavities 20 a, 20 b, 20 c and 20 d on the diaphragm 32 are toreduce the mass the diaphragm for better high frequency response. Thetransducer has no backplate since its diaphragm 32 is suspended on thecavity 40 of the carrier wafer 12.

By comparing the actuation embodiment described above with the sensingembodiment described previously, certain differences can be noted. Inthe actuation embodiment, the external peripheral edge is within theinterior peripheral edge of the cavity 12, such that the diaphragm 32only partially covers the cavity 12. In addition, the fixed comb fingers35 are taller than was the case previously. These differences areintended to improve performance in the actuation mode, as will bedescribed in more detail below.

Manufacture

FIG. 2-FIG. 11 show the major process steps that may be used tomanufacture either a sensing or actuation device.

The general steps to manufacture the capacitive transducer includefirst, applying a first etching mask on a layer that is mounted on asubstrate to define the position of one of a movable set of fingers anda fixed set of fingers. The position of the body and the springs mayalso be defined by the first mask. Next, a second etching mask isapplied to define the movable set of fingers, the fixed set of fingers,a body, and springs, the body being connected to the movable set offingers and the springs, the movable set of fingers being interdigitatedwith the fixed set of fingers. The second etching mask is then used toetch the layer and the first etching mask. The second etching mask isremoved, and the layer is then etched using the first etching mask, suchthat one of the movable set of fingers and the fixed set of fingers isshorter than the other of the movable set of fingers and the fixed setof fingers. The body, the springs, and the movable set of fingers arethen released using etching, such that, upon applying a force to thebody, the body moves parallel to the substrate. The variations in thisprocess to obtain the various embodiments will be apparent from thediscussion below.

FIG. 2 shows the wafer for the transducer. The process for making suchwafer is not described here. The layer 10 is a layer of conductivematerial such as preferred single crystalline bulk silicon or low stresspolysilicon. The layer 11 is a layer of dielectrical materials such asan oxide or nitride. Carrier wafer 12 material can be regular silicon orglass. The substrate can also be purchased from any SOI (Silicon OnInsulator) vendors. Although a variety of materials can be used, SOItype wafer is used for the purpose of explaining the process of a firstembodiment.

FIG. 3 shows the SOI wafer after growing a layer of oxide 13 and 16 onits top and bottom sides. The thermal oxidation process can be used foroxide growth. FIG. 4 shows the substrate after being subjected to asilicon anisotropic etch in KOH (Potassium hydroxide) or TMAH(Tetramethylammonium hydroxide) and, subsequent oxide etching in abuffered HF(Hydrofluoric acid) solution with the top side of the SOIwafer protected. The cavity 14 is formed on the oxide layer 11 and acavity 40 on the carrier silicon wafer 12. The cavity 40 can be alsoetched using any other anisotropic etching methods such as Silicon DeepReactive Ion Etching (DRIE).

The critical processing for making the vertical comb driver structure isto guarantee the perfect alignment of movable fingers 36 with the fixedfingers 35. If they are misaligned, the air gaps between one moveablefinger with adjacent two fixed fingers or vice visa will not be equal,which will result in the sideway movement of the movable finger 36 dueto unequal electrostatic force on its right and left side. Thisunexpected sideway movement will cause the malfunction of the comb drivestructure.

The manufacturing steps applies the self alignment process tomicrofabricate the vertical comb drive structure. FIG. 5 shows thepatterning of the oxide layer on the top side of a SOI wafer. The oxidepattering is done using regular lithography and oxide etching processsuch as RIE (Reactive Ion Etch). The oxide 22 on the movable fingers ispatterned. Areas 17 a, 17 b, 17 c, 17 d, 18 a, 18 b, 18 c, and 18 d arepattered for the anchors 37 and 38. Areas 21 a, 21 b and 21 c arepatterned for the electrical interconnection structure 39. Areas 201 a,201 b, 201 c and 201 d are oxide free for formation silicon cavities 20on the diaphragm 32.

FIG. 6 a shows the SOI wafer depicted in FIG. 5 after photoresistpatterning. This lithography step defines the shape of movable fingersand hinges, and redefines the oxide pattern depicted in FIG. 5. In orderto accommodate the possible larger alignment tolerance during theprocess, the geometry sizes of oxide patterning in FIG. 5 are largerthan the desired device feature sizes. An enlarged perspective view of aportion C of the comb fingers 25 and 27 and hinge 26 depicted in FIG. 6a is shown in FIG. 6 b. The final shapes of the movable comb fingers 36,fixed comb fingers 35, diaphragm 32 and hinges 29 a, 29 b, 29 c, and 29d are defied precisely by photoresist 25, 27, 23 and 26 respectively.The extra oxide 24 of oxide layer 13 will be cleared out by a subsequentoxide RIE process. FIG. 7 shows the SOI wafer in FIG. 6 after oxide RIEetch process. An enlarged perspective view of a portion D of the combfingers 25 and 27 and hinge 26 depicted in FIG. 7 a is shown in FIG. 7b.

The patterned photoresist layer is used as etching mask material for thefirst silicon DRIE etching. The oxide layer 11 is used as the etchingstop layer for the first silicon DRIE. FIG. 8 shows the substrate afterthe first silicon DRIE.

The photoresist is removed after the first silicon DRIE etching. FIG. 9b shows an enlarged perspective view of a portion E of the comb fingers28 and 30 and hinge 29 a depicted in FIG. 9 a. The hinge 29 a and fixedcomb finger 28 have no oxide on their tops while the movable finger 30and diaphragm 32 have oxide on their tops for the subsequent secondsilicon DRIE etching. The second silicon DRIE etching forms the lowerfixed comb fingers, flexible hinges 29 a, 29 b, 29 c and 29 d andcavities 20. FIG. 10 b shows an enlarged perspective view of a portion Fof the comb fingers 28 and 30 and a hinge 29 a depicted in FIG. 10 a.

After etched in buffered HF, the diaphragm 32, hinges 29 a, 29 b, 29 cand 29 d, and movable comb fingers 36 are released form the oxide layer11. The finished transducer is shown in FIG. 11 and FIG. 1. Themicrofabrication processes disclosed here doesn't include the processesto integrate with standard IC COMS process. However, it is very easy toachieve such integration for the people skill in the art. FIG. 12 showsa cross sectional perspective view of the microphone according to asecond embodiment, where the comb finger configuration is different. InFIG. 12, the movable fingers 35 and fixed fingers 36 are offset, whereasin FIGS. 1 and 11, the movable fingers 35 are higher than the fixedfingers 36. Either set of fingers may be taller or positioned above theother. While the offset fingers are more difficult to manufacture, theyhave a larger effective range motion, or in the actuation mode, requireless power.

Referring to FIG. 13, an air gap 41 between comb fingers 35 and 36 andan air gap 42 between hinges 29 a, 29 b, 29 c, and 29 d and comb fingers36 of around 2 μm provides sufficient resistance for a low frequencyresponse of the transducer. If 2 μm is achievable for air gaps 41 and 42with the current microfabrication technology, then the long air flowpath 33 shown in between diaphragm 32 and the carrier wafer 12 becomesunnecessary.

If the device is intended to be used as a microspeaker, a largertranslation of the diaphragm 32 during actuation is preferable to createa higher sound wave pressure level from the miniature siliconmicrospeaker. So a thicker silicon layer 10 as shown in FIG. 14 shouldbe used to make the larger height difference between the fixed fingers35 and movable fingers 36. In doing so, a larger electrostatic force anda correspondingly larger actuated translation movement between the fixedand movable fingers 35 and 36 can be expected. A larger silicon cavity40 should also be formed in the carrier wafer 12 so that the diaphragm32 can obtain a larger up and down translational movement without anymechanical obstruction. The miniature microspeaker embodiment isdepicted in the FIG. 14.

One of the advantages of silicon microspeaker is that it offers lesspower consumption due to electrostatic actuation. In addition, for thesame hinge design, the driving voltage for the silicon microspeaker canbe further lowered by reducing the overlapping region between fixed andmovable comb fingers, such that they are offset. The reason for this isthat the electrical field in the overlapping region between fixed andmovable comb fingers 35 and 36 will prevent the constructive movement ofthe diaphragm 32. One way to reduce the overlapping region between fixedand movable comb fingers 35 and 36 is to etch away the lower part of thefixed comb fingers 35 during the fabrication of the engineered SOIwafer. For example, the device layer may be pre etched before it isbonded with a carrier silicon wafer. FIGS. 15 and 16 depict embodimentswhere the lower portion of either fixed comb finger 35 or movable finger36 is etched away.

Another alternative embodiment is shown in FIG. 20, and formed of an Ntype substrate with P type structures. A regular N type silicon wafer 18in FIG. 17 is the starting material for the transducer. A layer of P++silicon 49 is formed on top of N type silicon 48 by either epitaxialgrowth or doping/diffusion or implantation/diffusion as shown in FIG.18. P++ silicon 49 is used for building the transducer. Referring toFIG. 19, the P++ silicon 49 is used as silicon etch stop layer to formdiaphragm 50 using silicon anisotropic etching in either KOH or TMAH,which will etch the N type substrate 18, but not the P++ silicon 49.

The embodiment shown in FIG. 20 is formed using the self alignmentprocess method disclosed above with reference to FIG. 3-FIG. 11. Thefixed comb finger 35 is electrically isolated from movable comb finger36 and diagram 32 by the PN junction formed between the N type and the Ptype layers. The transducer made based on this embodiment will reducethe wafer cost and increase the flexibility for integration with IC CMOSprocess.

In this patent document, the word “comprising” is used in itsnon-limiting sense to mean that items following the word are included,but items not specifically mentioned are not excluded. A reference to anelement by the indefinite article “a” does not exclude the possibilitythat more than one of the element is present, unless the context clearlyrequires that there be one and only one of the elements.

It will be apparent to one skilled in the art that modifications may bemade to the illustrated embodiment without departing from the spirit andscope defined in the Claims.

1. A method of manufacturing a capacitive transducer comprising thesteps of: applying a first etching mask on a layer to define theposition of one of a movable set of fingers and a fixed set of fingers,the layer being carried by a substrate; applying a second etching maskto define the movable set of fingers, the fixed set of fingers, a body,and springs, the body being connected to the movable set of fingers andthe springs, the movable set of fingers being interdigitated with thefixed set of fingers; etching the layer and the first etching mask usingthe second etching mask; removing the second etching mask; etching thelayer using the etched first etching mask such that one of the movableset of fingers and the fixed set of fingers is shorter than the other ofthe movable set of fingers and the fixed set of fingers; and releasingthe body, the springs, and the movable set of fingers using etching,such that, upon applying a force to the body, the body moves parallel tothe substrate.
 2. The method of claim 1, wherein etching comprises DeepReactive Ion Etching (DRIE).
 3. The method of claim 1, wherein the layeris a silicon wafer and the silicon wafer is bonded to the substrateusing one of fusion bonding, anodic bonding and epoxy bonding.
 4. Themethod of claim 1, wherein the layer is a layer of p-type material andthe substrate is of n-type material, and wherein the layer is formed onthe substrate by one of doping, implantation, and deposition.
 5. Themethod of claim 1, wherein the position of the body and the springs isdefined by the first etching mask.
 6. The method of claim 1, furthercomprising the step of, prior to mounting the layer to the substrate,etching a face of the layer that engages the substrate.
 7. The method ofclaim 1, further comprising the step of depositing a lightweightmaterial on the body.
 8. The method of claim 7, further comprising thestep of using a polymer as the lightweight material.